1. Field of the Invention
The present invention relates to a preamplifier circuit, a clock switching circuit, and an optical receiver implementing such circuits. Particularly, the present invention relates to a preamplifier circuit for amplifying a signal that is supplied to an optical receiver and is electrically converted through optical-to-electric conversion, and a clock switching circuit for switching a clock according to the transmission speed of an input signal.
2. Description of the Related Art
Exemplary configurations of a preamplifier circuit that amplifies a signal that is supplied to an optical receiver and is electrically converted through optical-to-electric conversion are disclosed in Japanese Patent Laid-Open Publication No. 3-195107 and Japanese Patent Laid-Open Publication No. 3-270504, for example. FIG. 1 is a block diagram showing an exemplary configuration of a preamplifier circuit according to the prior art. The illustrated preamplifier circuit includes a photo detector 1, a preamplifier 2, a variable resistive element 3, comparator circuits 4 and 5, and a control voltage generating circuit 6. The photo detector 1 converts an optical input signal into an electric signal through optical-to-electrical conversion. In turn, the preamplifier amplifies the output signal from the photo detector 1. The variable resistive element 3 is arranged to be parallel with the preamplifier 2 and its resistance may vary according to a bias signal applied thereto. The output signal from the preamplifier 2 is supplied to a next stage circuit and the comparator circuits 4 and 5.
The comparator circuit 4 compares the output signal from the preamplifier 2 with a first standard voltage and supplies the comparison result to the control voltage generating circuit 6. The comparator circuit 5 compares the output signal from the preamplifier 2 with a second standard voltage that is lower than the first standard voltage and supplies the comparison result to the control voltage generating circuit 6. The control voltage generating circuit 6 calculates an exclusive OR signal (i.e., rise and fall detection signal) from the comparison results, calculates an average value voltage of the exclusive OR signal, conducts differential amplification on the average value voltage and a standard voltage, and outputs the resulting amplified voltage as a control voltage. This control voltage is used as a bias voltage to be applied to the variable resistive element 3 to change the resistance of the variable resistive element 3.
In this way, the variable resistive element 3 may control the amount of voltage from the preamplifier 2 output that is to be fed back to the input side to control the gain and frequency bandwidth of the preamplifier 2 and adjust the frequency of the optical input signal to a suitable value.
However, in the preamplifier circuit of the prior art the number of rises and falls of the input signal within a certain time period must be within a predetermined range. Thereby, when the number of rises and falls of the optical input signal is below the predetermined range, the control voltage of the variable resistive element 3 may not be properly generated and band control may not be properly conducted.
It is desired that an optical receiver be capable of accurately realizing the so-called 3R functions, namely, reshaping, retiming, and regenerating a signal light.
FIG. 2 is a block diagram showing an exemplary configuration of an optical receiver according to the prior art. It is noted that components shown in this drawing that are identical to those shown in FIG. 1 are given the same numerical notations. It is also noted that the comparator circuits 4 and 5 and the control voltage generating circuit 6 are omitted from this drawing. In the illustrated optical receiver, the photo detector 1 converts an optical input signal into an electric signal through optical-to-electric conversion. The output signal of the photo detector 1 is amplified by the preamplifier 2. The variable resistive element 3 is arranged to be parallel with the preamplifier 2, and its resistance may be changed by a bias voltage.
The output signal from the preamplifier 2 passes through an AGC (automatic gain control) amplifier 7 to be supplied to a clock generating circuit 8 and an identification circuit 9. The clock generating circuit 8 generates a clock that is in sync with a clock component included in the output signal from the AGC amplifier 7 using a PLL or SAW filter and supplies the generated clock to the identification circuit 9. The identification circuit 9 samples the output signal from the AGC amplifier 7 using the clock from the clock generating circuit 8, reproduces the sampled data, and outputs the data along with the clock.
The optical receiver according to the prior art is designed in accordance with the transmission speed of the signal light being used so as to properly realize the 3R functions. For example, dedicated optical receivers are designed for transmission speeds of 622 Mb/s, 2.48 Gb/s, and 10 Gb/s, respectively.
In such case, for example, if the preamplifier 2 and the AGC amplifier 7 that realize equalizing amplification are designed for a transmission speed of 2.48 Gb/s, when a signal that has a higher transmission speed than the equalizing band (2.48 Gb/s) such as a transmission signal of 10 Gb/s is received at the optical receiver, the received signal (10 Gb/s) may be subject to band restriction at the equalizing amplifiers thereby resulting in deformation of the waveform and degradation of the reception characteristics of the signal. Also, when a signal that has a lower transmission speed than the equalizing band such as a transmission signal of 622 Mb/s is received at the optical receiver, a high frequency noise component of the receives signal may be increased thereby resulting in degradation of the reception characteristics.
As can be appreciated from the above descriptions, a preamplifier that is adapted for the transmission speed of the signal light being used needs to be implemented, and accordingly, in order to realize a structure for receiving signals with transmission speeds of 622 Mb/s, 2.48 Gb/s, and 10 Gb/s, respectively, three types of optical receivers are needed. However, such a structure results in an increase in the circuit size and manufacturing cost.